cmd/godoc: initialize filesystem and metadata for -url

Unlike when using -http, godoc -url didn't initialize the "filesystem"
and metadata that are used when generating responses. This CL adds this
initialization, so that -url provides the same results as an HTTP
request when using -http.

Fixes #4335.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/6817075
1 file changed